The FbInit() function is required for drivers with accelerated graphics support. It is used to replace default cfb.banked functions with accelerated chip-specific versions. vga256LowlevFuncs is a struct containing a list of functions which can be replaced. This struct defined in vga256.h. Examples of FbInit() functions can be found in the et4000, pvga1 and cirrus drivers.
If you do NOT need this function, simply don't define it, and put `NoopDDA' in its place in the vgaVideoChipRec structure initialization.